Intruder detection system utilizing artificial ambient light

ABSTRACT

The detection system includes a pair of photocells connected in series to form a voltage divider. One photocell is responsive to ambient light (supplied by either an incandescent or fluorescent lamp) and the other to reference light (supplied by a small auxiliary incandescent lamp). Inasmuch as the system is responsive to artificial ambient light, which contains an alternating component by reason of being supplied by the usual 60 hertz power source, any spurious changes in the supply voltage are nullified by the voltage divider since my detection system is also connected to the same source as that supplying the artificial ambient light. The signal forwarded from the voltage divider comprising the two photocells is filtered, amplified, rectified, and the resulting output signal employed to actuate a suitable alarm, this being when an intruder has caused a change in the amount of artificial light impinging upon the photocell responsive to ambient light. The system is insensitive to changes in ambient light derived from sources other than the alternating current power supply, such as changes in daylight or headlights on passing automobiles.

United States Patent [191 Corbin Jan. 7, 1975 [541 INTRUDER DETECTION SYSTEM UTILIZING ARTIFICIAL AMBIENT LIGHT [57] ABSTRACT [76] Inventor: Patrick L. Corbin, 13109 Circle Dr.,

Burnsville, Minn. 55337 [22] Filed: Feb. 26, 1973 [21] Appl. No.: 335,804

[52] US. Cl. 340/258 B, 250/221, 340/276 [51] Int. Cl. G08b 13/18 [58] Field of Search 340/258 B, 258 D, 228 S; 250/221 [56] References Cited UNITED STATES PATENTS 2,269,340 l/l942 Gulliksen 340/258 B X 2,912,683 11/1959 Bagno 340/258 B 3,255,441 6/1966 Goodwin et al. 340/220 3,278,923 10/1966 Archer 340/228 S X 3,370,284 2/1968 Bagno 340/258 B 3,444,544 5/1969 Pearson et al 340/258 B 3,603,957 9/1971 Merchant 250/221 X Primary Examiner-David L. Trafton Attorney, Agent, or Firm-Stuart R. Peterson The detection system includes a pair of photocells connected in series to form a voltage divider; One photocell is responsive to ambient light (supplied by either an incandescent or fluorescent lamp) and the other to reference light (supplied by a small auxiliary incandescent lamp). Inasmuch as the system is responsive to artificial ambient light, which contains an alternating component by reason of being supplied by the usual 60 hertz power source, any spurious changesin the supply voltage are nullified by the voltage divider since my detection system is also connected to the same source as that supplying the artificial ambient light. The signal forwarded from the voltage divider comprising the two photocells is filtered, amplified, rectified, and the resulting output signal employed to actuate a suitable alarm, this being when an intruder has caused a change in the amount of artificial light impinging upon the photocell responsive to ambient light. The system is insensitive to changes in ambient light derived from sources other than the alternating current power supply, such as changes in daylight or headlights on passing automobiles.

9 Claims, 2 Drawing Figures Patented Jan. 7, 1975 INTRUDER DETECTION SYSTEM UTILIZING ARTIFICIAL AMBIENT LIGHT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to intruder detection systems, and pertains more particularly to one responsive only to ambient light derived from an incandescent or fluorescent lamp supplied from an alternating current power source.

2. Description of the Prior Art Numerous attempts have been made in the past to devise satisfactory intruder detection systems. The more basic type involves an interruption ofa light beam so that it no longer reaches the photocell that normally picks up this light. The shortcomings of such a system have been recognized and, at least, one attempt to intensity-modulate the transmitted light has been made. Phase relationships have also been resorted to but these require more elaborate equipment and are not completely reliable, especially in view of extraneous noise conditions that may at times arise.

While some of the prior art systems have met with a reasonable degree of success, especially those of a rather sophisticated character, there still exist deficiencies in the systems with which I am acquainted that render such systems objectionable, due to their cost, the ability to detect them when in operation and/or the inability to provide a reliable alarm, such as when there has been a sudden change in light conditions not attributable to an intruder.

SUMMARY OF THE INVENTION Accordingly, a general object of my invention is to provide an intruderdetection system that will be responsive only to artificial ambient light changes, more specifically changesoccurring in the level of light provided by either an incandescent or fluorescent lamp when changed by the presence of an unauthorized person.

Another object is to provide a detection system that is self-contained, there being no external transmitters as such. Also, the invention has for an aim the elimination ofnot only any beam-type transmitter but also the avoidance of the coding that has heretofore been employed in order to makesuch a transmitter more foolproof.

Another object is to provide a system that is not dependent upon absolute values of light intensities, the system responding to rapid changes in light intensity and being insensitive to gradual changes such as those occurring from a change in the suns position, specifically the rising of the sun in the morning.

Also, the invention has for an object the provision of a simple system requiring no complex or timeconsurning adjustments in readying it for operation. More specifically, the invention has for an aim the provision of only a single external control in the form of an adjustment knob that can be easily manipulated in order to select an optimum detection range for the prevailing light conditions.

A further object is to provide a system of the foregoing character that does not depend on wall-reflected light, thereby enabling the system to respond more readily to the passage of a person in the more immediate vicinity of my system, thereby rendering the system independent of light reflected from walls that would be beyond a normal detection range.

Yet another object is to provide a compact system that can be contained in a very small casing, thereby contributing to its being unobstrusive. Coupled with lack of any visible transmitter or light beam, my system is indeed quite difficult to detect and circumvent.

An object is to provide a system that is quite inexpensive as well as simple, thus encouraging its purchase and use.

Still another object of the invention is to provide a detection system that will be virtually immune to changes in line voltages. Prior art devices have been made to turn off for a period oftime following a voltage fluctuation, but my system remains active and fully capable of detection throughout the fluctuation period.

Another important object of the invention is to provide a system that will detect both positive and negative light intensity excursions as far as the norm is concerned, the simple'expedient of wearing dark'clothes not enabling the system to be foiled.

Briefly, my intruder detection system comprises a pair of photocells connected in series to provide a voltage divider. By having one photocell function as a primary sensor, being located at the focus of a parabolic reflector, it senses all ambient light. in a room including that from an incandescent or fluorescent lamp, whereas the second photocell serves as a reference device, nullifying any spurious or sudden changes in voltage that might occur. The lamp for the reference photocell not only supplies light to this reference device but additionally serves as an on-off visual indicator to the user. When there is a change of light impinging on the primary sensor, there is a change in the voltage at the junction between the two photoelectric sensors.

A capacitor connected to the junction between the two photocells blocks the direct current component of the signal but permits the alternating current or fluctuating component containing the hertzsignal to be passed, this being derived from the: alternating component contained in the artificial ambient light, more specificallythat from an incandescent lamp or a pair offluorescent lamps energized from a conventional 60' cycle power source. It will be appreciated that fluorescent lamps are normally connected for out-of-phase operation which produces the same 120 hertz frequency experienced with a single incandescent lamp. After appropriate amplification, an alternating or fluctuating signal of proper magnitude is provided as a result ofthe passage of the previously mentioned 120 hertz signal through the capacitor. Any direct current voltage provided by the primary sensor from steady state light conditions, of course, is filtered out or blocked by reason of the capacitors capability to pass only the 120 hertz signal. While a changing voltage signal is derived from the system due to the movement of an unauthorized person, it is derived only from the alternating current component of the ambient light source. Therefore, the change is indicative of the change in that light striking the primary sensor, which change is caused by the movement of an intruder in the room where my system is being used. The changing signal, if it reaches the magnitude for which my system has been adjusted, triggers a silicon controlled switch so that a relay is picked up which sounds an alarm, such as a bell or buzzer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a fragmentary perspective view of an artificially lighted room having my intruder detection system set up for operation therein, and

FIG. 2 is a schematic diagram depicting the various electrical components utilized in the circuit constituting my system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, a fragmentary portion of a typical room has been pictured containing the usual concealed wall wiring denoted generally by the reference numeral 12. Two electrical outlets l4, 16 have been shown. Additionally, a toggle-type wall switch 18 has been illustrated, it serving to turn on a conventional overhead lamp (not shown), either of the incandescent or fluorescent variety. The artificial ambient light in the illustrated situation is provided by a conventional incandescent bulb or lamp 20 mounted on a lamp base 22, a portion of the lamp shade 24 having been broken away to reveal this lamp 22. The usual flexible cord 26 in this instance has its pronged plug 28 connected to the wall outlet 14. As will be better understood as the description progresses, the lamp 20 provides the artificial ambient light in this particular instance, although it might very well in practice be derived from an overhead lamp (not shown) controlled by the wall switch 18. The lamp base 22 rests on a table 30. Another item of furniture in the room 10 is a bookcase 32 containing books 34 and, for example, a stereo set 35; the use to which the bookcase 32 is put will be described presently.

Describing now my intruder detection system, denoted generally by the reference numeral 36, it will be observed from FIG. 1 that the entire system is contained in a casing 38 having an opening 40 in the front wall thereof. A parabolic reflector 42 can be viewed through the opening 40. As will be better understood hereinafter, the reflector 42 simply concentrates the ambient light so that it is focused on a photosensor yet to be referred to. In the top wall of the casing 38 is a lens 44 that passes light to indicate when the system 36 is operational. A knob 46 appears at one side of the casing 38, it being used to adjust the threshold level at which my system 36 is triggered into action. Extending from the casing 38 is an electrical cord 48 having a pronged plug 50 thereon.

Turning now to a description of FIG. 2, it will be noted from this figure that my intruder detection system 36 comprises four separate modules: (I) a power supply 52; (2) a sensor-filter 54; (3) an amplifier-' detector 56, and (4) a thresholder or output circuit 58.

The power supply 52 includes the previously mentioned electrical cord 48 and its plug 50. Additionally, there is a step-down transformer 59 composed of a primary winding 60 and two secondary windings 62a, 62b. The secondary winding 62a connects to a first bridge rectifier 640, whereas the secondary winding 62b connects to a similar bridge rectifier 64b. The rectified voltage in each instance is supplied to a filter, the filter associated with the rectifier 64a being labeled 66a and the filter associated with the rectifier 64b being labeled 66b. Both filters 66a, 66b provide a low ripple DC voltage. There is an output terminal 68a for the filter 66a, the other side simply being grounded. With respect to ground, the terminal 68a is normally at +12 volts. Similarly, the filter 66b has an output terminal 68b associated therewith, the other side of this filter likewise being grounded. The terminal 68b is normally at l2 volts. Still further, there are two additional output terminals 70, 72 which supply an alternating 12 volts. lnasmuch as a conventional household power supply is to be utilized when practicing the invention, [17 volts AC is made use of for the operation of my intruder detection system 36, the transformer 59 simply stepping down or lowering the household voltage to the usable 12 volts AC that my system 36 is designed to operate Passing now to a description of the sensor-filter 54, it will first be discerned that it includes an input terminal 78. Two photo-sensing devices in the form of photocells 80, 82 are connected in series to form a voltage divider having ajunction at 84. ln other words, the photocell is in one leg of the divider and the photocell 82 in the other leg thereof. As will presently be made manifest, the junction 84 between the two photocells 80, 82 provides a voltage signal that will be forwarded for further processing the rest of my system 36.

At this particular stage, though, it is important to understand that the photocell 80 is a primary sensor and that it is located at the focal point of the previously mentioned parabolic reflector 42, the parabolic reflector 42 being superimposed upon FIG. 2 in phantom outline. The photocell 82 functions as a reference device and is situated within the casing 38, being proximally located with respect to an auxiliary lamp 86. The lamp 86 is connected to a pair of terminals 88, 90 which are in turn connected to the previously mentioned terminals 70, 72 of the power supply module 52 so as to be energized at 12 volts. Not only does the auxiliary lamp 86 provide a reference level of light intensity, which is concomitantly influenced by any voltage excursions experienced by the household power supplied to the room 10, but it serves as a convenient onoff visual indicator to the user of my system. In this latter regard, a portion of the light emitted by the lamp 86 passes through the previously mentioned lens 44 shown in FIG. 1. v

A filter, more specifically, a capacitor 94 is connected at one side to the junction 84 between the voltage divider provided by the photocells 80 and 82; this capacitor 94 blocks the passage ofdirect current. Additionally, it will be discerned that a resistor 96 extends from the other side of the capacitor 94 to ground. The R-C time constant provided by the resistor 96 and capacitor 94 is fairly low, thereby rendering the system insensitive to real slow signal changes. It is, of course, the 120 hertz component of the signal picked up by the photocell 80 that is significant and is passed on to the amplifier-detector 56 via the capacitor 94. Direct current components are not wanted and would be grossly midleading if forwarded; the capacitor 94 assures they are not.

At this time, a description will be given of the amplifier-detector 56. It will be observed that it includes a coupling resistor 98, this resistor being connected to the previously mentioned capacitor and resistor 94, 96. An operational amplifier indicated generally by the reference numeral 100 has a pair of power inputs 100a, 100b, a pair of signal inputs 100e, 100d, an output l00e and an adjustable feedback resistor l00fin the form of a potentiometer controlled or set through the agency of the knob 46 shown in FIG. 1. While operational amplifiers are now quite common, it might be mentioned that the power inputs 100a and 1011b receive power from thepower supply 52, there being conductors extending from the terminals 68a, 68b. More specifically, a +12 volts is applied to the power input 100a and a -12 volts to the power input 10012. The resistor 98 is actually connected, as is evident from FIG. 2, to the signal input 1000, which is the inverting input, the other signal input 100d being connected to ground through a resistor 101. To provide impedance matching, the value of the resistor 101 should approximate the sum of that for the resistors 96 and 98. It can be mentioned in passing that the ratio of the resistance of resistor l00f to that of resistor 98 determines the gain of the operational amplifier 100. More will be said hereinafter concerning the gain.

The output signal from the output 100e of the opera- I tional amplifier 100 is delivered to an isolating transformer 102 having a primary winding 104 to which the output 100:: is connected and a secondary winding 106. The isolating transformer 102 also functions to step up the signal voltage at this stage, the secondary winding 106 having more turns thereon than the primary winding 104.

The stepped up voltage from the transformer 102 is fed to a bridge rectifier 108 which converts the alternating current voltage from the secondary winding 106 to an equivalent direct current voltage, doing so in conjunction with an attached filter 110 composed ofa first resistor 112, a second resistor 114 connected to ground, and a capacitor 1l6paralleling the resistor 114.

Owing to the rectifying action supplied by the bridge rectifier 108 and the filtering action supplied by the filter 110, a voltage signal reflecting any change in the intensity of the alternating light striking the photocell 80 is provided. The filter 110 merely smoothes out this voltage, minimizing the ripples therein, and furnishes the direct current voltage that is proportional to the AC component of the light impinging on the sensor 80 as just mentioned.

' Connected to the filter 110 is a capacitor 118 which couples the resulting signal from the filter 110 to a resistor 120, there also being a resistor 122 leading to ground; as with the resistor 96 and capacitor 94, the resistor 122 and capacitor 1113 provide a low R-C time constant so that any slow changes are not reflected in the final output. The resistor 120 is connected to a second operational amplifier 124 which has a pair of power inputs 124a, 124b, the input 124a being connected to the +12 volt terminal 68a of the power supply 52 and the input 124b to the -12 volt terminal 68b. More specifically, the resistor 120 is connected to the non-inverting input 124d of the amplifier 124, the other input 124a being connected to ground. Theoutputof the amplifier 124 has been labeled l24e. There is a feedback resistor l24f; the ratio of the value of resistor 124f to that of the resistor 125 determines the gain of the amplifier 124, the gain being fixed in this instance in contra-distinction to the earler-mentioned gain of the amplifier 100. There is also a resistor 125 leading to ground which, together with the resistors 124f and 120 provide a desired impedance match for the amplifier 124.

The output signal from the amplifier 124 is fed to a resistor 126 leading to ground.

It will be seen that the thresholder or output circuit 58 includes a terminal 128 connected to the +12 volt terminal 68a. The output 124e, however, is electrically attached to a resistor 130 that is in turn connected to a silicon controlled switch 132, more specifically a silicon controlled rectifier, having an anode 132a, cathode 1321) and a gate 1320, the resistor 130 actually being connected to the gate [326 so as to apply a triggering voltage to this gate and thereby cause the switch 132 to become closed and thereby allow current to flow through its anode 132a and cathode l32b to ground.

A relay 134 is employed having a coil 134a in the anode circuit of the silicon controlled switch 132. The relay 134 further includes normally open contacts 134); that are closed when the coil 134a is energized, the energization of the coil 134a depending upon the arming or gating of the silicon controlled switch 132. A manual reset switch 136, which is normally closed, is connected between the coil 134a of the relay 134 and the previously mentioned terminal 128. The switch 136 is opened in order to cause the switch 132 to become reverse-biased or open.

In circuit with the contacts l34b of the relay 134 is an appropriate alarm, such as a bell or buzzer 140. It will be appreciated, though, that the alarm 140 may constitute various forms, even being located at a remote vantage point, such as at a surveillance station located some distance away and connected by telephone lines.

TABLE OF PRINCIPAL COMPONENTS Components that have been used in a working embodiment of my invention and which might be helpful to identify are:

Photocells 80, 82 Clajrex 903A Operational amplifiers 100,124 Model 741C Silicon Controlled Rectifier 132 GE C103A I Capacitor 94,118 5 at Capacitor 116 10 uf Resistor 96 1K Resistors 101.125 5.6K Resistors 98,120 4.7K Resistor 112 100 Ohms Resistor 114 10K Resistor 122 560 Ohms Resistor 126 500 Ohms Resistors 124f,l30 100K Potentiometer 1001 2M OPERATION Having presented the foregoing descriptiomthe manner in which my intruder detection system 36 operates should be readily understandable. However, a brief recapitulation will perhaps be helpful in appreciating the full benefits to be derived from a system of this type. Assuming that the casing 38 containing my system 36 has been unobtrusively placed in the room 10, such as 'on one of the shelves of the bookcase32 and further concealed by flanking books 34, which is to be protected or safeguarded and that the plug 50 has been inserted into the wall outlet 16, the next step is to turn on the lamp 29, if it has not already been turned on, (or close the switch 18 in order to energize a ceiling lamp if this is preferred). Although exposed to limited view in FIG. 1, it will be understood that the casing 38 may be placed in almost any inconspicuous location, even behind an item of furniture, such as a chair, but somewhere where its field of view embraces the area'to be protected. The lamp does not have to be in the field of view; all that is required is that the field of view be illuminated by some of the light supplied by the lamp 20. Thus, only a very small portion of the total light emanating from the lamp 20 need be received, particularly in view of the use of the parabolic reflector 42 which functions to focus any light rays entering the opening 40 onto the primary photocell 80. With the power being supplied to the system 36 via the cord 48 and plug 50, a small fraction of the light from the auxiliary lamp 86 is made visible through the lens 44 in order to indicate that the system is operational.

With the preliminary steps just mentioned taken care of, the next step is to adjust the value of the potentiometer or feedback resistor 100f, and hence the gain of the operational amplifier 100, doing so by manipulating the adjusting knob 46 (FIG. 1) on the side of the cabinet 38 which is connected to the wiper arm (FIG. 2) of the resistor 100]". The proper gain, effected with the knob 46, is needed in order to assure that an adequate threshold triggering voltage will be supplied to the silicon controlled switch 132, more specifically to the gate 1320. The proper setting can be easily achieved by having someone walk in the vicinity of the casing 38. One person can take care of the setting if need be by merely making an initial adjustment of the gain via the knob 46 and then walking in the vicinity of the casing 38, successively repeating this procedure to whatever extent is necessary in order to derive the proper actuation of the alarm 140 for whatever ambient light condition then prevails within the field of view of my system 36.

With my system conditioned as described above, it is simply left unattended and when an intruder passes in the vicinity of the system 36, that is, within its field of view, a change in the light striking the primary sensor or photocell 80 is instrumental in changing the resistance of this sensor 80. It will be appreciated that under these particular circumstances only the resistance of photocell 80 is modified, there being no change with respect to that of the photocell 82. Consequently, there is a change in voltage at the junction 84 between the two photocells 80, 82 which is manifested in the magnitude of thesignal passing through the capacitor 94. The capacitor 94 passes the 120 hertz but not any direct current, so only the change in light derived from the lamp 20 is forwarded, direct current signals being blocked by the capacitor 94. Other artificial lighting, such as from an overhead lamp, may be in the room 10, and the system would react to the composite amount of artificial lightting, but it does so only in response to alternating or fluctuating components derived from the 60 cycle power frequency.

Thus, when the photocell 80 changes its resistance, there is a variation in the output at the junction 84 of the voltage divider comprised of the two sensors 80, 82. On the other hand, should both photocells change together there would be no significant change in the output at the junction. In other words, when the alternating light conditions seen by the sensor 80 change, an output is produced, but if ,a change occurs in the line voltage, both photocells react the same because the lamp 20 will have its light output change in accordance with the changing voltage, and the auxiliary lamp 86, receiving power from the same supply, will also change and thus there is no output from the voltage divider comprised of the two sensors 80, 82. This latter feature is important in order to obviate the chance of having false alarms caused by local line voltage disturbances. Such disturbances can be caused by, for example, the automatic starting up of the blower motor on the household furnace.

At any rate, assuming that the change has been sensed only by the photocell 80, the capacitor 94 passes the resulting hz signal (but not any zero frequency or DC signal), forwarding this fluctuating signal to the amplifier-detector 56. This signal is delivered to the inverting input 100C of the operational amplifier 100. Hence, an output signal at the output 100e of this amplifier is forwarded to the transformer 102 and the signal from its secondary winding 106 is impressed on the bridge rectifier 108. After smoothing out the ripple in the rectified signal from the bridge rectifier 108, this being done by the filter 110, the resulting DC voltage is proportional to the AC component of the impinging light. This DC voltage will be constant if there is no change in the level of light striking the photocell 80. On the other hand, it will be a changing DC voltage if the light striking the photocell 80 is changing (as it will do due to the passage of an intruder). The DC voltage, when changing, is really an AC signal which can be passed by the capacitor 118 to the input 124d ofthe operational amplifier 124. Stated somewhat differently, it is differentiated by the components 118 and 122, the R-C time constant being relatively low so as to ignore slow changes, and the resulting voltage applied to the input 124d. The output from the amplifier 124 is the signal used to trigger the silicon controlled switch 132, assuming that it is of sufficient amplitude to do so. It will be recalled that the resistive value of the feedback resistor l00f is set or adjusted by way of the knob 46 so as to produce a sufficient voltage in accordance with the passage of a person relative to my intruder detection system 36, more specifically with respect to the photovell 80 disposed at the focal point of the parabolic reflector 42, so as to sound the alarm 140.

Consequently, if the signal delivered tothe gate 1320 is sufficiently large, the switch 132 will become conductive and there will be enough current flowing through the relay coil 134a of the relay 134 to pick up or close the normally open contacts 134b, closure of these contacts energizing the bell or buzzer which can be used to frighten away the intruder and/or to notify neighbors and the police. As already mentioned, a signal can be forewarded or telemetered to a central station for the purpose of alerting appropriate personnel to the unauthorized intrusion.

It is important to recognize that the entire system 36 lends itself readily to miniaturization. In an actual embodiment of the invention, the dimensions of the casing 38 were 7 inch X 7 inch X 4 inch, thereby helping my system to go unnoticed by unauthorized persons. Of course, the salient advantage stems from the fact that no beam-type transmitted is employed which would be looked for by most intruders. In this latter regard, it will be appreciated that it has become quite customary to leave at least one light on when residents of a household are away. Such precautionary measures have become known to would-be burglars, and they simply resort to various schemes in order to learn of the absence of occupants from a dwelling. Once in a lighted room, containing my system 36, more specifically within the systems field of view, their presence is quickly detected and the alarm 140 sounded. If per chance the intruder does not enter the field of view embraced by my system and decides to turn off the lamp 20, this has the same result (actually more pronounced) as if he had entered the field of view, for the reference lamp 86 will still be energized with the consequence that a large signal is produced at the junction 84. Hence, under these conditions the alarm 140 will also be sounded.

I claim:

1. An intruder system responsive to artificial ambient light derived at least in part from a 60 cycle alternating power supply, the system comprising a photosensor disposed so as to provide an electric signal having a value representative of the amount of ambient light impinging thereon which signal contains a fluctuating component in accordance with the frequency of said power supply, means controlled only by a sufficient change in value of the fluctuating component of said electric signal to provide an output signal, a reference lamp energized from said power supply, a second photosensor disposed so as to receive light only from said reference lamp to provide an electric signal having a value representative of the amount of reference light impinging thereon, said photosensors being connected in series with a junction therebetween, and said controlled means being connected to said junction, whereby the system is sensitive to a change in ambient light derived only from said alternating current power supply when caused by an intruder and is insensitive to changes in ambient light derived from sources other than said power supply.

2. An intruder detection system comprising a photocell, a capacitor having one side connected to said photocell so as to forward an electric signal containing an alternating component variable in accordance with the frequency of artificial light derived from an alternating current power source and to block any signal derived from generally steady-state ambient light, an operational amplifier having a pair of input terminals and a feedback potentiometer to vary the gain of said amplifier, one of which terminals inverts the signal applied thereto, the output side of said capacitor being connected to said inverting terminal, an isolation transformer having its primary winding connected to the output terminal of said operational amplifier and having a secondary winding, a rectifier bridge connected to said secondary winding, a differentiating network connected to said rectifier bridge for passing a signal when the output from said rectifying bridge is changing, a second operational amplifier having a pair of input terminals, one of which is a non-inverting terminal, said non-inverting terminal being connected to said differentiating network, a resistor connected to the output terminal of said second operational amplifier, a threshold device connected to said resistor for providing an output signal when the signal obtained from said resistor is of sufficient magnitude as determined by the feedback potentiometer associated with said first amplifier, and an alarm connected to said threshold device which is energized when said threshold device is triggered into operation.

3. A system in accordance with claim 2 including a second photocell connected in series with said first photocell to provide a junction therebetween, and an auxiliary lamp connected to said alternating power source for supplying reference light to said second photocell, said capacitor being connected to said junction, whereby spurious voltages from said power source are nullified at said junction and are not transmitted through said capacitor.

4. A system in accordance with claim 3 in which said auxiliary lamp also functions as an indicating lamp.

5. A system in accordance with claim 3 including a parabolic reflector, said first photocell being located at the focal point of said reflector and said reflector having an opening via which ambient light enters to strike said first photocell.

6. An intruder system responsive to artificial ambient light at least partially provided by a lamp connected to an alternating power supply, the system comprising a first photosensor disposed so as to provide an electric signal having a value representative of the amount of ambient light impinging thereon which signal contains a fluctuating component in accordancewith the frequency of said power supply, a second photosensor disposed so as to provide an electric signal having a value representative of the amount of reference light impinging thereon, said photosensors being connected in series with a junction therebetween, an auxiliary lamp disposed relative to said second photosensor to provide said reference light, said auxiliary lamp also being connected to said alternating current supply, and means connected to said junction controlled only by sufficient change in value of the fluctuating component of the electric signal provided by said first photosensor to provide an output signal, whereby the system is sensitive to a change in ambient light derived from said alternating current power supply when caused by an intruder and is insensitive to changes in ambient light derived from sources other than said power supply.

7. A system in accordance with claim 6 including a I casing, said auxiliary lamp being contained in said cas- 8. A system in accordance with claim 7 in which a portion of the light from said auxiliary lamp passes through a lens in said casing so as to indicate when the system is operational.

9. An intruder system responsive to artifical ambient light derived at least in part from a conventional alternating power supply having a 60 hertz frequency, the system comprising a photosensor for receiving at least a portion of said ambient light, means in circuit with said photosensor for passing only a hertz signal derived from said 60 hertz power supply which varies in magnitude depending upon the magnitude of said ambient light portion received by said photosensor, an alarm, means connected to said passing means for activating said alarm when said 120 hertz signal is below a given magnitude, an auxiliary lamp for connection to said power supply so as to furnish a reference level-of light intensity, and a second photosensor responsive to only said reference level of light, said passing means being connected to both of said photosensors so that voltage changes in said power supply concomitantly influence both of said photosensors so as not to affect the magnitude of said 120 hertz signal by reason of any such voltage changes. 

1. An intruder system responsive to artificial ambient light derived at least in part from a 60 cycle alternating power supply, the system comprising a photosensor disposed so as to provide an electric signal having a value representative of the amount of ambient light impinging thereon which signal contains a fluctuating component in accordance with the frequency of said power supply, means controlled only by a sufficient change in value of the fluctuating component of said electric signal to provide an output signal, a reference lamp energized from said power supply, a second photosensor disposed so as to receive light only from said reference lamp to provide an electric signal having a value representative of the amount of reference light impinging thereon, said photosensors being connected in series with a junction therebetween, and said controlled means being connected to said junction, whereby the system is sensitive to a change in ambient light derived only from said alternating current power supply when caused by an intruder and is insensitive to changes in ambient light derived from sources other than said power supply.
 2. An intruder detection system comprising a photocell, a capacitor having one side connected to said photocell so as to forward an electric signal containing an alternating component variable in accordance with the frequency of artificial light derived from an alternating current power source and to block any signal derived from generally steady-state ambient light, an operational amplifier having a pair of input terminals and a feedback potentiometer to vary the gain of said amplifier, one of which terminals inverts the signal applied thereto, the output side of said capacitor being connected to said inverting terminal, an isolation transformer having its primary winding connected to the output terminal of said operational amplifier and having a secondary winding, a rectifier bridge connected to said secondary winding, a differentiating network connected to said rectifier bridge for passing a signal when the output from said rectifying bridge is changing, a second operational amplifier having a pair of input terminals, one of which is a non-inverting terminal, said non-inverting terminal being connected to said differentiating network, a resistor connected to the output terminal of said second operational amplifier, a threshold device connected to said resistor for providing an output signal when the signal obtained from said resistor is of sufficient magnitude as determined by the feedback potentiometer associated with said first amplifier, and an alarm connected to said threshold device which is energized when said threshold device is triggered into operation.
 3. A system in accordance with claim 2 including a second photocell connected in series with said first photocell to provide a junction therebetween, and an auxiliary lamp connected to said alternating power source for supplying reference light to said second photocell, said capacitor being connected to said junction, whereby spurious voltages from said power source are nullified at said junction and are not transmitted through said capacitor.
 4. A system in accordance with claim 3 in which said auxiliary lamp also functions as an indicating lamp.
 5. A system in accordance with claim 3 including a parabolic reflector, said first photocell being located at the focal point of said reflector and said reflector having an opening via which ambient light enters to strike said first photocell.
 6. An intruder system responsive to artificial ambient light at least partially provided by a lamp connected to an alternating power supply, the system comprising a first photosensor disposed so as to provide an electric signal having a value representative of the amount of ambient light impinging thereon which signal contains a fluctuating component in accordance with the frequency of said power supply, a second photosensor disposed so as to provide an electric signal having a value representative of the amount of reference light impinging thereon, said photosensors being connected in series with a junction therebetween, an auxiliary lamp disposed relative to said second photosensor to provide said reference light, said auxiliary lamp also being connected to said alternating current supply, and means connected to said junction controlled only by sufficient change in value of the fluctuating component of the electric signal provided by said first photosensor to provide an output signal, whereby the system is sensitive to a change in ambient light derived from said alternating current power supply when caused by an intruder and is insensitive to changes in ambient light derived from sources other than said power supply.
 7. A system in accordance with claim 6 including a casing, said auxiliary lamp being contained in said casing.
 8. A system in accordance with claim 7 in which a portion of the light from said auxiliary lamp passes through a lens in said casing so as to indicate when the system is operational.
 9. An intruder system responsive to artifical ambient light derived at least in part from a conventional alternating power supply having a 60 hertz frequency, the system comprising a photosensor for receiving at least a portion of said ambient light, means in circuit with said photosensor for passing only a 120 hertz signal derived from said 60 hertz power supply which varies in magnitude depending upon the magnitude of said ambient light portion received by said photosensor, an alarm, means connected to said passing means for activating said alarm when said 120 hertz signal is below a given magnitude, an auxiliary lamp for connection to said power supply so as to furnish a reference level of light intensity, and a second photosensor responsive to only said reference level of light, said passing means being connected to both of said photosensors so that voltage changes in said power supply concomitantly influence both of said photosensors so as not to affect the magnitude of said 120 Hertz signal by reason of any such voltage changes. 